Prepaid cards should be denominated in RP

It's hard to decide whether I want a prepaid RP card or not when I don't know how many RP it's worth, and when I'm in a place like Walmart where none of the staff I could ask would know. Thus, it's never going to be an impulse buy for me. Why not just denominate them in RP? After all, the price of RP probably won't change much between the time Riot sells the card wholesale and the time I activate it.